Struggling to remember which ion goes where, or how to balance those half equations? Let’s break it down – fast. With less than a week to go before GCSE exams, it’s easy to feel overwhelmed. Electrolysis is one of those topics that students often find confusing – half equations, predicting products, and understanding what happens at each electrode can feel like a lot to remember. But don’t worry – with a clear approach, you can master the key ideas quickly.

📝 Note: This guide is written specifically for the AQA GCSE Chemistry and Combined Science specifications, with Higher Tier content clearly marked.

What Is Electrolysis?

Electrolysis is the process of passing electricity through a molten or aqueous ionic compound (electrolyte). The ions move to the electrodes where they are discharged to form elements.

Discharged: In electrolysis, this means an ion reaches an electrode and either gains or loses electrons so that it turns into a neutral element or compound. For example, a copper ion gets electrons at the cathode to become copper metal.

The Essentials You Need to Know

⚡ Ions must be free to move – this means the compound must be molten or dissolved in water (aqueous).
🧲 Anode vs Cathode – negative ions go to the anode (+), positive ions go to the cathode (–).
📌 PANIC – Positive is Anode, Negative is Cathode.
🔁 OIL RIG – Oxidation Is Loss, Reduction Is Gain (of electrons) → Higher Tier only

What Happens at Each Electrode?

🧪 Higher Tier only: Explaining oxidation and reduction in terms of electrons and writing half equations.

Electrode	Type of Ion Attracted	Reaction Type	Example Half Equation (HT only)
Cathode (–)	Positive ions (cations)	Reduction	Cu²⁺ + 2e⁻ → Cu
Anode (+)	Negative ions (anions)	Oxidation	2Cl⁻ → Cl₂ + 2e⁻

Remember: OIL RIG and PANIC help you keep it straight in the exam.

Reactivity Series – Why It Matters

In aqueous solutions, sometimes hydrogen (from H⁺ ions) or oxygen (from OH⁻ ions in water) is discharged instead of the metal or non-metal ion. To decide this, you need to know the reactivity series:

Quick Reactivity Guide (most reactive to least):
Potassium > Sodium > Calcium > Magnesium > Aluminium > Carbon > Zinc > Iron > Tin > Lead > Hydrogen > Copper > Silver > Gold

Mnemonic: Please Stop Calling Me A Careless Zebra Instead Try Learning How Copper Saves Gold

At the cathode, you compare the metal ion to hydrogen:

If the metal is more reactive than hydrogen, hydrogen is discharged.
If the metal is less reactive than hydrogen, the metal is discharged.

⚠️ Important Tip: Only three common metals are less reactive than hydrogen: copper, silver, and gold.
That means:
🔹 If the solution contains Cu²⁺, Ag⁺, or Au³⁺, the metal will form.
🔹 If not, then hydrogen is discharged.

Example: In aqueous copper(II) sulphate, copper is less reactive than hydrogen → copper is produced at the cathode.

How to Predict the Products

If the compound is molten:

The metal forms at the cathode.
The non-metal forms at the anode.

If the compound is aqueous:

Use the reactivity series for the cathode (hydrogen vs metal).
At the anode:

If the solution contains a halide (Cl⁻, Br⁻, I⁻), that halogen is released.
Otherwise, oxygen is released from OH⁻ ions in water.

How Electrolysis Questions Are Asked in Exams

Expect questions like:

Predict the product at each electrode.
Describe what is seen during electrolysis.
(Higher Tier only) Write half equations and identify oxidation or reduction.

Exam Tips:

✅ Always balance charges in half equations (HT only)
✅ Include state symbols if asked
✅ Clearly label your electrodes in diagrams
✅ Focus on fewer high-quality examples rather than endless notes

🧠 Electrolysis: Can You Remember These?

✔️ Tick off what you can do without looking at your notes:

⬜ I know what PANIC and OIL RIG mean
⬜ I can explain why ions move during electrolysis
⬜ I can describe what happens at the cathode and anode
⬜ I can name the 3 metals less reactive than hydrogen
⬜ I can predict the products of aqueous and molten electrolysis
⬜ I know what to look for at the anode if no halide is present
⬜ I can write at least one correct half equation (HT only)

📝 If you didn’t tick them all — scroll back and review. If you did — great work! Time to try a practice question.

Practice Question (with Answer)

Practice Question
Q: A solution of copper(II) sulphate is electrolysed using inert electrodes.
What forms at each electrode? Write half equations and state the type of reaction.

Cathode: Copper is less reactive than hydrogen → copper forms.
Half Equation (HT only): Cu²⁺ + 2e⁻ → Cu (Reduction)

Anode: Sulphate is not a halide → oxygen forms.
Half Equation (HT only): 4OH⁻ → O₂ + 2H₂O + 4e⁻ (Oxidation)

🧪 Required Practical: Electrolysis of Aqueous Solutions (AQA RP3)

This topic links directly to Required Practical 3 in the AQA GCSE Chemistry and Combined Science courses.

You may be asked to:

Predict the products of electrolysis for a given solution
Describe what is seen at each electrode (e.g. gas bubbles, copper coating, colour changes)
Write half equations (for Higher Tier students)

Common test solutions include:

Copper(II) sulphate
Sodium chloride (brine)

📘 Example 1: Electrolysis of Aqueous Sodium Chloride (Brine)

Ions present:

Na⁺, Cl⁻ (from NaCl)
H⁺, OH⁻ (from water)

Products:

Cathode: H⁺ → H₂ gas (because H⁺ is less reactive than Na⁺)
Anode: Cl⁻ → Cl₂ gas (halide, so preferred over OH⁻)
Left in solution: Na⁺ and OH⁻ → sodium hydroxide (NaOH)

Electrodes are normally inert materials, such as carbon or platinum, which don’t react during the electrolysis and simply allow the electric current to pass through.

Half-equations (HT only)

⚡ At the Cathode (negative electrode):

Hydrogen ions (from water) are reduced (RIG): 2H⁺ + 2e⁻ → H₂

⚡ At the Anode (positive electrode):

Chloride ions are oxidised (OIL): 2Cl⁻ → Cl₂ + 2e⁻

✅ Third Main Product:

Even though it’s not released at an electrode, sodium hydroxide is the third main product.

How do you know?

From the equations: Na⁺ and OH⁻ are not discharged
They remain in solution and form NaOH
NaOH is an alkali → turns red litmus paper blue

🔍 Exam-Style Question

Q: What is the third main product of the electrolysis of brine, and how could it be detected?

A:
The third main product is sodium hydroxide (NaOH). It can be detected by placing a drop of the solution on red litmus paper, which will turn blue, showing that an alkali is present.

📘 Example 2: Electrolysis of Aqueous Copper(II) Sulphate

Ions present:
Cu²⁺, SO₄²⁻, H⁺, OH⁻

Products:

Cathode: Cu²⁺ → Cu (copper metal forms)
Anode: OH⁻ → O₂ gas
In solution: H⁺ + SO₄²⁻ → dilute sulphuric acid (H₂SO₄)

Half-equations (HT only)

⚡ At the Cathode (negative electrode):

Copper ions are reduced (RIG): Cu²⁺ + 2e⁻ → Cu

⚡ At the Anode (positive electrode):

Hydroxide ions are oxidised (OIL): 4OH⁻ → O₂ + 2H₂O + 4e⁻

✅ So the third main product, after charges are balanced, is sulphuric acid, left behind in the solution.

🔍 Exam-Style Question

Q: A solution of copper(II) sulfate is electrolysed using inert electrodes.
Identify the third product that forms and explain how it is detected.

A:
Copper forms at the cathode and oxygen at the anode.
The remaining ions in solution are H⁺ and SO₄²⁻, which form dilute sulphuric acid.
This is the third product, although it is not released at an electrode. It lowers the pH of the remaining solution. Therefore, Blue litmus paper turns red in the presence of an acid.

🔬 Triple Science Only: Fuel Cells

This topic is part of the Separate Science course (not required in Combined Science).

Fuel cells are a type of electrical cell that produce electricity from a chemical reaction between hydrogen and oxygen. The only waste product is water.

All Triple students should know:

Fuel cells produce electricity continuously if fuel is supplied.
They are used in spacecraft, vehicles, and energy-efficient devices.
They only produce water as waste.

Higher Tier students also need to write the half equations:

At the anode (oxidation):
2H₂ → 4H⁺ + 4e⁻

At the cathode (reduction):
O₂ + 4H⁺ + 4e⁻ → 2H₂O

Overall reaction:
2H₂ + O₂ → 2H₂O

⚙️ Electrolysis or Carbon Reduction – Which Method?

Whether a metal is extracted by electrolysis or by heating with carbon depends on its position in the reactivity series.

🔽 Metals below carbon - Use Carbon Reduction

(e.g. zinc, iron, tin, lead)
✅ Can be extracted by reduction with carbon
✅ Carbon displaces the metal from its oxide
✅ Cheaper and more energy-efficient

🔼 Metals above carbon - Use Electrolysis

(e.g. aluminium, magnesium, calcium)
❌ Cannot be extracted by carbon – carbon is not reactive enough
✅ Must be extracted by electrolysis which uses electricity to split the molten compound into elements.
❌ More expensive – requires electricity and high temperatures

Case Study: Aluminium Extraction

Aluminium is extracted from aluminium oxide (Al₂O₃) using electrolysis.
But aluminium oxide has a very high melting point (over 2000°C).

To reduce energy costs, cryolite is used:

🔹 Cryolite lowers the melting point of aluminium oxide
🔹 This makes the process more energy-efficient and less expensive

During electrolysis:

Al³⁺ ions move to the cathode and are reduced to aluminium
O²⁻ ions move to the anode and are oxidised to oxygen

🧪 Higher Tier only:
Al³⁺ + 3e⁻ → Al (reduction)
2O²⁻ → O₂ + 4e⁻ (oxidation)

What happens to the oxygen?
If the anode is made of carbon and is hot, the oxygen produced can react with it to form carbon dioxide (CO₂):

C + O₂ → CO₂

As a result, the anode wears away over time and needs to be replaced.

📘 Exam-Style Question

Q: Aluminium is extracted by electrolysis, but iron is extracted by heating with carbon. Explain why different methods are used.

A:
Aluminium is more reactive than carbon, so carbon cannot displace it from its oxide. It must be extracted using electrolysis, which uses electricity to break down molten aluminium oxide.
Iron is less reactive than carbon, so it can be extracted by heating with carbon, which is a cheaper and more energy-efficient method.

🧠 Electrolysis in 5 Quick Reminders

PANIC – Positive is Anode, Negative is Cathode
OIL RIG (HT only) – Oxidation Is Loss, Reduction Is Gain
Molten = Metal + Non-metal
Aqueous = Reactivity series + Halide rule
Practice writing half equations for full marks (HT)

Final Tips

✅ Foundation: focus on what forms where
✅ Higher Tier: include half equations and identify oxidation/reduction
✅ Use clear, exam-style explanations – don’t just memorise

🟡 Get Full Marks in the Exam:

Apply content accurately under pressure
→ Practice applying these ideas to unfamiliar scenarios (e.g. different compounds, unseen ions).
Use precise language, especially for 4–6 mark explain questions
→ You’ll need to use words like:

attracted to the cathode
more reactive than hydrogen
oxidised by losing electrons

Label state symbols if asked (HT only)
→ You must remember to include them.
Master exam technique - reading the question carefully, managing time, checking for multiple parts

📥 Flashcards covering all key points from this topic are available to download in the resources section of the website.

Need help with tricky topics like electrolysis or fuel cells? TutorAnt offers expert one-to-one support in GCSE Science to help you feel confident and prepared. Book a session today!

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Tuesday, 6 May 2025

Electrolysis Made Easy: A Last-Minute Guide to Acing These Questions